When did eukaryotic cells (cells with nuclei and other internal organelles)
first evolve? What do we know about how they evolved from earlier
life-forms?

The origin of the eukaryotes--the
kingdom of life that includes all of the higher plants and animals,
including ourselves--took place in the heavily obscured early history of the
earth. Consequently, there is still much speculation involved in answering
this question. Carl
Woese, a professor of microbiology at the University of Illinois at
Urbana-Champaign and the discoverer of archaebacteria, offers one reply:

"Evidence from microfossils strongly suggests that life arose on the earth
long ago, probably within a few hundred million years of the planet's
formation. Sedimentary rocks 3.5 billion years old (and perhaps those 3.8
billion years old) contain what appear to be fossil stromatolites,
which are natural colonies formed by photosynthetic bacteria; within the
stromalites one can see microscopic forms reminiscent of bacteria. If these
presumed bacteria are direct ancestors of extant photosynthetic bacteria,
life was already well developed by then, having passed through the stages
that led to the most recent universal ancestor and the splitting of the
ancestral lineage into the primary lines of descent.

"By comparing molecular sequences to infer genealogies, molecular
phylogeneticists tell us that the two primary lines of descent lead to the
eubacteria (or common bacteria, which include the photosynthetic bacteria)
and to a second common lineage that subsequently divided to form the archaea
(which like the eubacteria are prokaryotes) and the eukaryotes (which
include all higher plants and animals). All these events appear to have
preceded the oldest fossil stromatolites. So the eukaryotic lineage appears
to be very ancient, about as ancient as the two prokaryotic lineages.

"The key unanswered question here concerns when on the eukaryotic line the
eukaryotic type of cell formed. Eukaryotic cells seem structurally far more
complex than their prokaryotic counterparts (from which they arose), so
biologists generally believe that many evolutionary steps must have
separated the two. Nevertheless, the eukaryotic stem on the phylogenetic
tree of life spawns many branches before one gets to the split that
separates the ancestors of plants from the ancestors of animals, which seems
to have happened more than a billion years ago. There seem to have been many
earlier branchings from the eukaryotic stem, all represented by unicellular
eukaryotes (such as the slime molds, the flagellates, the trichomonads, the
diplomonads, the microsporidia, among others).

"Clearly, eukaryotic history goes back far into the era when the earth's
atmosphere contained little or no oxygen, well over two billion years ago.
Our current concept of the origin of the eukaryotic cell is in flux,
however, and an evolutionary sequence that appears simple when
conceptualized on a phylogenetic tree diagram may be far more complex and
interesting in reality. We know that the eukaryotic cell is of ancient
origin, but we do not yet know the evolutionary dynamic that underlies its
formation."

J. Peter Gogarten in the department of molecular and cell biology at the
University of Connecticut at Storrs, gives a broader overview:

"The question is the subject of an ongoing and lively controversy. The best
guesses for the time when eukaryotes evolved range from just below 2.0
billion years to around 3.5 billion years before the present.

"One of the less ambiguous sources of information is the fossil record. Work
by Gonzalo Vidal of the University of Uppsala
in Sweden indicates that single-celled planktonic eukaryotes certainly date
back to 1.7 billion years B.P. and very likely to at least 2.2 billion years
B.P. The early fossil record is very sparse, however, and small eukaryotic
cells present in the fossil record would not necessarily have been
positively identified. My colleagues generally agree that the fossil record
provides only a most recent estimate for the time when eukaryotes were
already abundant; they might have been around a long time before they made
it into the fossil record in a recognizable form.

"Another approach is to date phylogenies by looking at a 'clock' of
molecular changes that accumulate in the genetic code. Although that
approach has been successfully used to decipher relations between organisms,
calibrating it to measure the time elapsed since the divergence of
phylogenetic branches is problematic; concerning the early evolution of
life, there is no generally accepted approach. Most attempts to date early
molecular phylogenetic trees used the emergence of eukaryotes (around 2.0
billion years B.P.) as a calibration point. Russell
F. Doolittle and his co-workers at the University of California at San
Diego recently attempted to extend the calibration further into the past,
but this work is contested. The controversy centers on possible cases of
horizontal gene transfer across phylogenetic branches that were ignored by
these authors and on an insufficient correction for multiple substitutions.
In addition, these backward extrapolations assume that the rate of molecular
change at the time the eukaryotes originated is the same as it was during
the metazoan evolution, when in fact it was probably much faster.

"The typical eukaryotic cell resulted from a symbiosis between different
prokaryotic ancestors. Three prokaryotic components can be traced by
comparing molecules in extant prokaryotes and eukaryotes. These components
are the mitochondria (derived from purple bacteria), the plastids (from
cyanobacteria), and the nucleocytoplasmic component (from archaebacteria).
Other features in eukaryotic cells--for instance, the cytoskeleton--may also
be of bacterial descent, but so far the molecular record has not yielded
unambiguous clues as to their origin.

"The nucleocytoplasmic component of the eukaryotic cell branches off very
early in the evolutionary radiation of the archaebacteria. There is an
active dispute as to whether some of the archaebacteria are more closely
related to the eukaryotic nucleocytoplasm than are others (proponents of the
differing views are James Lake of the University of California at Los
Angeles and Carl Woese of the University of Illinois at Urbana-Champaign).
Regardless of how the debate is resolved, the ancestor of the eukaryotic
nucleocytoplasm must have separated from the archaebacteria early in, or
even before, the era when the major archaebacterial groups arose. In
contrast, bacterial ancestors of mitochondria and plastids separated from
the eubacteria lineage only after the evolutionary radiation that gave rise
to the major eubacterial kingdoms. Therefore, it is likely that primitive
eukaryotes lacking mitochondria and plastids were around a long time before
they made it into the fossil record. I would be surprised if such eukaryotes
did not date back to at least 3.0 billion years before the present.

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